Two-photon absorption heteroaromatic chromophores and compositions thereof

ABSTRACT

The present invention relates to new heteroaromatic compounds with high two-photon absorption activity, useful in particular as optical power limiting agents via two-photon absorption or as imaging agents in confocal laser scanning fluorescence microsopy via two-photon absorption or excitation.

The present invention relates to new heteroaromatic chromophores withsignificant two-photon absorption activity.

It is known that molecules exhibit a nonlinear optical (NLO) behaviourby simultaneously absorbing two or more photons, either of the same orof different energy, to be promoted to one of their excited states whenexposed to an intense laser pulse. In the case of two-photon absorption(TPA), the frequency of the nonlinear absorption is approximately halfof that corresponding to the conventional linear one-photon absorption.As a consequence, the TPA frequency typically falls in the visiblered-near infrared (NIR) region of the electromagnetic radiationspectrum, where the material is transparent with respect to one-photonabsorption. TPA is a 3rd order NLO process and is described by theimaginary part of the 3rd order nonlinear susceptibility.

Once the molecule has reached one of its excited states via TPA, it mayshow a fluorescence emission to return to its ground-state. Inparticular, the two-photon induced fluorescence emission occurs at afrequency very similar to the one-photon induced fluorescence emission.The direct consequence of this phenomenon is that the two-photon excitedfluorescence frequency is usually larger than the TPA frequency, asopposed to the case of linear absorption, where the emitted frequency isalways smaller than that absorbed. Therefore, TPA dyes may absorb a redor NIR radiation (low frequency) and emit in the visible range.

Organic molecules able to show a significant TPA activity are veryimportant for a variety of emerging applications including opticallimiting (eye and sensor protection), three-dimensional opticalmemories, two-photon laser scanning fluorescence microscopy,up-converted lasing, non-destructive imaging of coated materials, andmicro- and nanofabrication (MEMS, microelectromechanical systems) (Denk,W.; Strickler, J. H.; Webb, W. W. Science 1990, 248, 73; Ehrlich, J. E.;Wu, X. L.; Lee, L. Y. S.; Hu, Z. Y.; Rockel, H.; Marder, S. R.; Perry,J. W. Opt. Lett. 1997, 22, 1843; Day, D.; Gu, M.; Smallridge, A. OptLett. 1999, 24, 948; Cumpston, B. H.; Ananthavel, S. P.; Barlow, S.;Dyer, D. L.; Ehrlich, J. E.; Erskine, L. L.; Heikal, A. A.; Kuebler, S.M.; Lee, I. Y. S.; McCord-Maughon, D.; Qin, J. Q.; Rockel, H.; Rumi, M.;Wu, X. L.; Marder, S. R.; Perry, J. W. Nature 1999, 398, 51; Belfield,K. D.; Ren, X. B.; Van Stryland, E. W.; Hagan, D. J.; Dubikovsky, V.;Miesak, E. J. J. Am. Chem. Soc. 2000, 122, 1217; Abbotto, A.; Beverina,L.; Bozio, R.; Bradamante, S.; Pagani, G. A.; Signorini, R. Synth. Met.2001, 121, 1755; Abbotto, A.; Beverina, L.; Bozio, R.; Bradamante, S.;Ferrante, C.; Pagani, G. A.; Signorini, R. Adv. Mater. 2000, 12, 1963).

The nonlinear absorption provides many advantages with respect to theconventional technologies based on linear absorption: a) two-photonexcitation occurs in the red or NIR region; this region overlaps withthe optical transparency window of cells and living tissues; as aconsequence, TPA provides much deeper light penetration depths asopposed to conventional techniques; b) the absorbed TPA intensity scalesquadratically with the intensity I of the incident laser radiation,which in turn decreases approximately as the square of the distance fromthe focus; the consequence is that molecules are excited via TPA only atthe focus of the beam; two-photon induced phenomena occur only at thefocus as well, with a fourth power increased spatial resolution; c) redand NIR light scattering is minimized with respect to higher frequencyradiation.

The TPA phenomenon has been theoretically predicted by Göppert-Mayer in1931 (Göppert-Mayer, M. Ann. Phys. 1931, 9, 273) and experimentallyconfirmed 30 years later (Kaiser, W. K.; Garrett, C. G. B. Phys. Rev.Lett. 1961, 7, 229). However, TPA has been studied in more detail onlywith the availability of proper laser sources. Moreover, all of TPAbased applications remained unexplored for decades due to the lack ofefficient TPA absorbers. Only recently a number of dyes exhibitingsignificant TPA activity have been proposed. The vast majority of thesemolecules are based on benzenoid derivatives substituted withconventional donor and acceptor groups such as NO₂, CN, SO_(n)R, CO₂R,OR e NR₂.

Few examples of TPA chromophores based on substituted heteroaromaticcompounds are known. Concerning this aspect, WO 01/70735 owned by thesame Applicant is mentioned.

In accordance with the present invention, new molecules are providedwith high TPA activity, via excitation with lasers operating in thevisible-red or NIR wavelength region, that is a range where most organicmolecular and polymeric materials and organic tissues are highlytransparent.

In accordance with the present invention, compounds are provided havingthe following general formulas (I) and (II)

wherein Het-1 and Het-3 are identical or different, and are selectedamong the following heterocyclic groups:

wherein Y may be O, S, or NZ with Z=H, lower alkyl, and aryl; andwherein R₅, R₆, R₇, R₈, and R₉ are the same or different, and areselected from the group consisting of H, alkyl groups having from 1 to18 carbon atoms, alkoxy, aminoalkyl, alkylhalide, hydroxyalkyl, alkylgroups containing hydroxy and amino functionalities, alkoxyalkyl,alkylsulfide, alkylthiol, alkylazide, alkylcarboxylic, alkylsulfonic,alkyylisocyanate, alkylisothiocyanate, alkylalkene, alkylalkyne, aryl,formyl, and that can contain electronpoor ethenylic moieties such asmaleimide, capable to react with nucleophilic groups such as —SH, andgroups such as isothiocyanate capable to react with groups such as —NH₂;and Het-2 is selected among the following heterocyclic groups:

wherein R₁₀ is selected from the group consisting of H, alkyl groupshaving from 1 to 18 carbon atoms, alkoxy, aminoalkyl, alkylhalide,hydroxyalkyl, alkyl groups containing hydroxy and amino functionalities,alkoxyalkyl, alkylsulfide, alkylthiol, alkylazide, alkylcarboxylic,alkylsulfonic, alkyylisocyanate, alkylisothiocyanate, alkylalkene,alkylalkyne, aryl, formyl, and that can contain electronpoor ethenylicmoieties such as maleimide, capable to react with nucleophilic groupssuch as —SH, and groups such as isothiocyanate capable to react withgroups such as —NH₂;and A is selected among the anions alkylsulfonate, arylsulfonate,polyarenesulfonate, triflate, halide, sulfate, methosulfate, phosphate,polyphosphate,and wherein n and m, the same or different may be 0, 1, 2;and R₁, R₂, R₃, and R₄, the same or different, may be H, lower alkyl,alkoxyalkyl, aryl, cyano, alkoxycarbonyl, —(CR₁₁R₁₂)_(p)-Het, wherein0<p<10, R₁₁ and R₁₂, the same or different, are selected from the groupof H, lower alkyl, and Het may be Het-1 or Het-2 or Het-3.

wherein Het-1, Het-3, and Het-4 are the same or different and areselected among the following heterocyclic groups:

wherein Y may be O, S, and NZ with Z=H, lower alkyl, aryl;and R₅ and R₆, are the same or different, and are selected from thegroup consisting of H, alkyl groups having from 1 to 18 carbon atoms,alkoxy, aminoalkyl, alkylhalide, hydroxyalkyl, alkyl groups containinghydroxy and amino functionalities, alkoxyalkyl, alkylsulfide,alkylthiol, alkylazide, alkylcarboxylic, alkylsulfonic,alkyylisocyanate, alkylisothiocyanate, alkylalkene, alkylalkyne, aryl,formyl, ketone, and that can contain electronpoor ethenylic moietiessuch as maleimide, capable to react with nucleophilic groups such as—SH, and groups such as isothiocyanate capable to react with groups suchas —NH₂; R₅, and R₆, the same or different, may further be the followingheterocyclic group:

and R₇, R₈, and R₉ are defined as above;and Het-2 is defined as above;and wherein n, m, p, and q, the same or different, may be 0, 1, or 2;and wherein R₁₃, R₁₄, R₁₅, R₁₆, R₁₇, R₁₈, R₁₉, and R₂₀ are the same ordifferent and are selected from the group of H, lower alkyl,alkoxyalkyl, aryl, cyano, alkoxycarbonyl, —(CR₂₁R₂₂)_(l)-Het, wherein0<l<10, and R₂₁ e R₂₂, the same or different, are selected from thegroup of H, lower alkyl, and Het may be Het-1 or Het-2 or Het-3, orHet-4.

For the uses according to the present invention said compounds can showtheir two-photon absorption activity as such, or once prepared insolution, or in a solid state.

In a further aspect of the present invention said compounds can beprocessed into compositions containing a polymer material such aspoly(methacrylate), polyimide, polyamic acid, polystyrene,polycarbonate, and polyurethane or an organically-modified silica (SiO₂)network.

In particular, in said compositions the said compounds are eitherdispersed or covalently bonded to the polymer materials or to the silicanetwork.

Features and advantages of the present invention will become readilyapparent by reference to the following detailed description inconjunction with the accompanying drawings, in which:

FIG. 1 shows a typical two-photon absorption profile of compound (3) inDMSO (dimethylsulfoxide) obtained via the open-aperture Z-scantechnique;

FIG. 2 shows a typical two-photon absorption profile of compound (6) inDMSO obtained with the same technique.

A detailed description of the invention is provided, with reference tocertain compounds, which possess a structure corresponding to theformulas (3), (6), and (7), with examples which are not limiting thepresent invention.

EXAMPLE 1

Compound (3), endowed with TPA properties, has been prepared by a triplecondensation reaction of compound (1) (Zhu, D.; Kochi, Jay K.Organometallics 1999, 18, 161) with an excess ofN-methyl-2-pyrrolecarboxaldehyde in refluxing n-butanol in the presenceof a catalytic amount of piperidine as a base.

N-methyl-2,4,6-[1-(N-methylpyrrol-2-yl)ethen-2-yl]pyridinium triflate(3). A solution of N-methyl-2-pyrrolecarboxaldehyde (1.545 g, 14.16mmol) in n-butanol (10 mL) was added dropwise to a solution ofN-methyl-sym-collidinium triflate (1.332 g, 4.42 mmol) in the samesolvent (40 mL). Ten drops of piperidine were added to the colorlesssolution and the mixture was stirred at reflux for 6 h. The resultingred-violet mixture was concentrated to ca. 15 mL and the red precipitatecollected under reduced pressure. The solid was washed with toluene (10mL) to give the product (1.783 g, 3.21 mmol, 68%) mp 250° C. (dec)(n-BuOH); ¹H-NMR (CDCl₃) δ 7.78 (2 H, s), 7.63 (1 H, d, J=16.0), 7.52 (2H, d, J=15.4), 6.90 (1 H, d, J=16.0), 6.82 (1 H, d, J=3.8), 6.78 (2 H,s), 6.75 (2 H, d, J=15.4), 6.75 (2 H, d, J=3.7), 6.72 (1 H, s), 6.21 (2H, d, J=3.2), 6.17 (1 H, d, J=3.2), 3.95 (3 H, s), 3.83 (6 H, s), 3.82(3 H, s); EA calcd for C₂₈H₂₉F₃N₄O₃S: C, 60.20%; H, 5.23%; N, 10.03%.Found: C, 60.60%; H, 5.29%; N, 9.62%.

We describe now a non limitative example related to a compound ofgeneral formula (I) and defined by the formula (6).

EXAMPLE2

Compound (6) has been prepared by a condensation reaction of aldehyde(5) (Akoudad, S.; Frere, P.; Mercier, N.; Roncali, J. J. Org. Chem.1999, 644267) and pyridinium salt (4) (Zhu, D.; Kochi, J. K.Organometallics 1999, 18, 161) in refluxing ethanol and in the presenceof a catalytic amount of piperidine.

N-methyl-2,6-[1-(3,4-ethilenedioxythiphen-2-yl)ethen-2-yl]pyridiniumtriflate (6). A solution of 3,4-ethylenedioxythiophene-2-carboxaldehyde(0.456 g, 2.7 mmol) in ethanol (10 ml) was added dropwise to a solutionof 1,2,6-trimethylpyridinium triflate (0.350 g, 1.3 mmol) and a fewdrops of piperidine in the same solvent (20 ml). Reaction mixture wasrefluxed for 3 hours and then cooled to 0° C. giving the formation of abrown-yellow precipitate that was filtered under reduced pressure andcrystallized from ethanol (0.539 g, 0.94 mmol, 72%). mp 103-105° C.¹H-NMR (DMSO-d₆) δ 8.24 (1 H, t, J=8.14), 8.16 (2 H, d, J=8.09), 7.66 (2H, d, J=15.54), 7.11 (2 H, d, J=15.63), 6.97 (2 H, s), 4.40 (4 H, m),4.29 (4 H, m), 4.11 (3 H, s); ¹³C-NMR (DMSO-d₆) 153.47 (2 C), 143.41 (2C), 142.28 (1 C), 142.02 (2 C), 131.50 (2 C), 126.40 (2 C), 122.64 (2C), 114.16 (2 C), 104.56 (2 C), 65.27 (2 C), 64.30 (2 C), 41.16 (2 C).Anal Calcd. for C₂₃H₂₀F₃NO₇S₃: C, 47.99%; H, 3.50%; N, 2.43%. Found: C,47.90%; H, 3.11%; N, 2.20%.

We describe now another non limitative example related to a compound ofgeneral formula (II) and defined by the formula (7).

EXAMPLE 3

Compound (7) has been prepared following a two-step procedure. In thefirst step the sim-collidinium salt (8) has been condensed with anexcess of aldehyde (9) (Abbotto, A.; Beverina, L.; Bozio, R.; Facchetti,A.; Ferrante, C.; Pagani, G. A.; Pedron, D.; Signorini, R. Org. Lett.,2002, 4, 1495) in hot propylene glycol and in the presence of catalyticpiperidine. Alkylation of the crude reaction product with an excess ofcetyltriflate (Abbotto, A.; Bradamante, S.; Facchetti, A.; Pagani, G. A.J. Org. Chem. 1997, 62, 5755) in anhydrous acetonitrile gave the puretitle compound.

N-cetyl-2,4,6-trimethylpyridinium triflate (8). A solution of cetyltriflate (1.873 g, 5 mmol) in dry toluene (5 ml) was added to a solutionof sym-collidine (0.606 g, 5 mmol) under dry atmosphere. The whitesolution was heated at about 60° C. for 1 hour and then solvent wasevaporated. The white residue was taken up with diethyl ether (10 ml)and filtered under reduced pressure, yielding the product as a whitesolid (1.982 g, 4 mmol, 80.0%) mp 54-56° C.

N-cetyl-2,4,6-[1-[N-methyl-5-(1-(pyrid-4-yl)-ethen-2-yl)pyrrol-2-yl]ethen-2-yl]pyridiniumtriflate (7). A solution of (9) (1.306 g, 6.1 mmol) in propylene glycol(15 mL) was added to a solution of N-cetyl-sym-collidinium triflate(0.460 mg, 0.93 mmol) and piperidine (5 drops) in the same solvent (10mL). The resulting orange mixture was heated at 130° C. for 6 h yieldinga dark violet solution which was cooled to room temperature and pouredinto Et₂O (100 mL). The obtained precipitate was collected by filtrationunder reduced pressure to give the monoquaternized precursor of (7) as ablack solid, which was washed with water (20 mL) and EtOH (5 mL) (0.270mg, 0.25 mmol, 27%) mp>350° C. (dec); ¹H NMR (DMSO-d₆) δ 8.53 (6 H, d,J=4.6), 8.23 (2 H, s), 7.99 (1 H, d, J=15.7), 7.75 (2 H, d, J=15.0),7.62 (2 H, d, J=16.2), 7.61 (1 H, d, J=15.0), 7.60 (1 H, d, J=14.8),7.58 (6 H, d, J=4.2), 7.26 (2 H, d, J=15.2), 7.17 (2 H, d, J=3.7), 7.10(3 H, d, J=15.9), 6.99 (1 H, d, J=4.2), 6.91 (3 H, m), 4.60 (2 H, t,broad), 3.96 (3 H, s), 3.95 (6 H, s), 1.78 (2 H, m, broad), 1.10-1.50(26 H, m), 0.82 (3 H, t, J=6.7); ¹³C NMR (DMSO-d₆) d 151.60 (2 C),149.94 (6 C), 149.93 (1 C), 144.43 (3 C), 136.22 (1 C), 135.99 (2 C),133.38 (1 C), 133.07 (2 C), 129.24 (2 C), 127.33 (1 C), 125.32 (1 C),125.17 (2 C), 121.09 (3 C), 120.54 (6 C), 119.68 (1 C), 117.79 (2 C),114.13 (2 C), 112.89 (2 C), 112.22 (1 C), 110.65 (1 C), 110.21 (2 C),49.95 (1 C), 31.26 (1 C), 30.83 (1 C), 30 73 (2 C), 28.50-29.20 (12 C),28.33 (1 C), 28.23 (1 C), 25.42 (1 C). A solution of cetyl triflate(2.050 g, 5.95 mmol) in anhyd. CH₃CN (40 mL) was added, under nitrogenatmosphere, to a solution of the product obtained as described in theprevious step (1.195 g, 1.11 mmol) in the same solvent (80 mL). Thereaction mixture was stirred overnight at room temperature and thesolvent evaporated to leave a residue which was taken up with Et₂O (30mL). The dark precipitate was collected by filtration under reducedpressure and washed several times with boiling hexane. The resultingblue solid was treated with boiling water to give the product (1.357 g,0.62 mmol, 55.9%). mp>350° C. (EtOH); ¹H NMR (DMSO-d₆) δ 8.80 (4 H, m),8.63 (2 H, m), 8.30 (2 H, s), 8.26 (1 H, d, J=14.1), 8.18-8.15 (4 H, m),8.02-7.96 (3 H, m) 7.92-7.86 (2 H, m), 7.82-7.74 (3 H, m), 7.39 (1 H, d,J=15.3), 7.31-7.24 (3 H, m), 7.23-7.14 (3 H, m), 7.12-7.09 (2 H, m),7.05 (1 H, m), 7.01 (1 H, m), 4.68 (2 H, t broad), 4.45 (6 H, t broad),3.98 (6 H, s), 3.97 (3 H, s), 1.90 (6 H, m broad), 1.77 (2 H, s broad),1.45-1.10 (104 H, m), 0.90-0.80 (12 H, m).

According to the present invention said compounds show large two-photonabsorption cross-sections both in solution and in the solid state. Wenow describe, with examples which are not limiting the presentinvention, experimental data of the two-photon absorption activity ofsaid compounds (3), (6), and (7).

We define the following parameters: β (two-photon absorptioncoefficient, concentration dependent), σ₂ and σ₂′ (cross-sections). Itis possible to obtain the absorption coefficient β by interpolation ofthe relationship between the transmittance T versus the initial laserbeam intensity I₀, in accordance with the following relationships:$T = {\frac{\ln\left( {1 + {I_{0}L\quad\beta}} \right)}{I_{o}L\quad\beta}\quad{where}}$$T = \frac{I_{t}}{I_{o}}$and L=1 cm and I_(t) is the intensity of the trasmitted beam. The I₀ andI_(t) dimensions are I_(t), I₀=[GW/cm²] whereas the β dimensions areβ=[cm/GW]

Since $\sigma_{2} = \frac{\beta^{\prime}}{N_{a}}$it follows that $\sigma_{2} = {\frac{\beta}{N_{a}c}10^{3}}$where N_(a) is the Avogadro's number and σ₂ has the dimensions of[cm⁴/GW].

Finally: σ′₂=hvσ₂. σ′₂ has the dimensions of$\left\lbrack \frac{{cm}^{4} \cdot s}{{photon} \cdot {molecule}} \right\rbrack.$

The following table summarizes the nonlinear optical characterizationdata for said compounds taken as examples. Compound λ(nm) Pulse duration(fs) Power (μJ) Intensity (GW/cm²) Concentration (mmoli/l) β(cm/GW)$\begin{matrix}{\sigma^{\prime}2} \\\left\lbrack \frac{10^{- 50}\quad{{cm}^{4} \cdot \quad s}}{{photon}\quad \cdot \quad{molecule}} \right\rbrack\end{matrix}\quad$ 3 785 130-150 0.14 100 29.0 0.078 113 6 785 130-1500.22 228 30.4 0.027 37 7 800 150 2.1 1600

FIGS. 1 and 2 show, as an example, the two-photon absorption activity ofcompounds (3) and (6), respectively. The TPA activity has beencharacterized by means of “open-aperture” Z-scan measurements of DMSOsolutions of the described compounds and with a laser source operatingat 780-790 nm with a pulse duration of 130-150 fs.

The Z-scan technique is one of the two most important experimentalprocedures to measure nonlinear absorption phenomena (two-photonabsorption). The open-aperture Z-scan enables the measurement of thenonlinear absorption of the sample by recording the transmittance T (theratio between transmitted and incident intensity) as a function of theincident intensity. To do this, the sample is moved along thepropagation direction (the Z axis) of a focused laser beam. The energyof the laser beam is kept constant, while the intensity grows up as thesample moves towards the focal plane (Z=0). Only the lineartransmittance contributes to the signal far from the focal plane. In theproximity of the focus the intensity grows up very quickly and thenonlinear absorption process generates a dip in the transmittance (T<1).The dip is symmetrical with respect to the position of the focal plane.When a fs source is employed, the Z-scan allows for the discriminationbetween simultaneous TPA and sequential multiphoton absorptionprocesses, involving intermediate excited states populated bynonradiative phenomena. In fact, in the case of fast (100-200 fs)pulses, the latter process does not contribute to the signal, being thenonradiative processes active in the picosecond (ps) or nanosecond (ns)regime. Since the two-photon absorption scales quadratically, and notlinearly, with the intensity of the incident laser radiation, themeasured absorption as a function of the incident intensity providesunequivocal evidence that the sample is a non-linear (two-photon)absorber. In this way the two-photon absorption parameters β and σ′₂ areexperimentally obtained.

FIGS. 1 and 2 show the Z-scan profiles for DMSO solutions of molecules(3) and (6), respectively, measured in a 1-mm cell and with pulse energyof 0.17 and 0.16 mJ. The normalized transmittance (I(z)/I(∞), where I(∞)is the transmitted intensity far from the focal plane) is plotted as afunction of the sample position (Z). The deep dip shown in both graphsis a clear evidence that a strong two-photon absorption is occurring insolution. In addition, the Figures ensure that the two compounds show nosignificant linear absorption at 785 nm and, therefore, are completelytransparent at low intensities of the incident radiation (Z far from thefocal plane). FIG. 1 proves that compound (3) shows a transmittanceT=0.77 at the focal point with a laser pulse energy of 0.17 mJ. Thisvalue is remarkably lower, that is the two-photon absorption is larger,than that obtained for other known molecules of comparable molecularweight using the same laser power and pulse width conditions.

In a further aspect of the present invention, in addition to opticallimiting activity, said compounds are useful for other applicationsbased on their two-photon absorption activity, such as use as imagingagents in confocal laser scanning fluorescence microscopy via two-photonabsorption or excitation.

1-16. (canceled)
 17. A compound of formula (I)

wherein Het-1 and Het-3 are identical or different, and are selectedamong the following heterocyclic groups:

wherein Y may be O, S, or NZ with Z=H, lower alkyl, and aryl; andwherein R₅, R₆, R₇, R₈, and R₉ are the same or different, and areselected from the group consisting of H, alkyl groups having from 1 to18 carbon atoms, alkoxy, aminoalkyl, alkylhalide, hydroxyalkyl, alkylgroups containing hydroxy and amino functionalities, alkoxyalkyl,alkylsulfide, alkylthiol, alkylazide, alkylcarboxylic, alkylsulfonic,alkylisocyanate, alkylisothiocyanate, alkylalkene, alkylalkyne, aryl,formyl, and that can contain electronpoor ethenylic moieties such asmaleimide, capable to react with nucleophilic groups such as —SH, andgroups such as isothiocyanate capable to react with groups such as —NH₂;and Het-2 is selected among the following heterocyclic groups:

wherein R₁₀ is selected from the group consisting of H, alkyl groupshaving from 1 to 18 carbon atoms, alkoxy, aminoalkyl, alkylhalide,hydroxyalkyl, alkyl groups containing hydroxy and amino functionalities,alkoxyalkyl, alkylsulfide, alkylthiol, alkylazide, alkylcarboxylic,alkylsulfonic, alkylisocyanate, alkylisothiocyanate, alkylalkene,alkylalkyne, aryl, formyl, and that can contain electronpoor ethenylicmoieties such as maleimide, capable to react with nucleophilic groupssuch as —SH, and groups such as isothiocyanate capable to react withgroups such as —NH₂; and A is selected among the anions alkylsulfonate,arylsulfonate, polyarenesulfonate, triflate, halide, sulfate,methosulfate, phosphate, polyphosphate; and wherein n and m, the same ordifferent may be 0, 1, 2; and R₁, R₂, R₃, and R₄, the same or different,may be H, lower alkyl, alkoxyalkyl, aryl, cyano, alkoxycarbonyl,—(CR₁₁R₁₂)_(p)-Het, wherein 0<p<10, R₁₁, and R₁₂, the same or different,are selected from the group of H, lower alkyl, and Het may be Het-1 orHet-2 or Het-3.
 18. A compound of formula (II)

wherein Het-1, Het-3, and Het-4 are the same or different and areselected among the following heterocyclic groups:

wherein Y may be O, S, and NZ with Z=H, lower alkyl, aryl; and R₅, andR₆, are the same or different, and are selected from the groupconsisting of H, alkyl groups having from 1 to 18 carbon atoms, alkoxy,aminoalkyl, alkylhalide, hydroxyalkyl, alkyl groups containing hydroxyand amino functionalities, alkoxyalkyl, alkylsulfide, alkylthiol,alkylazide, alkylcarboxylic, alkylsulfonic, alkylisocyanate,alkylisothiocyanate, alkylalkene, alkylalkyne, aryl, formyl, ketone, andthat can contain electronpoor ethenylic moieties such as maleimide,capable to react with nucleophilic groups such as —SH, and groups suchas isothiocyanate capable to react with groups such as —NH₂; R₅, and R₆,the same or different, may further be the following heterocyclic group:

and R₇, R₈, and R₉ are defined as in claim 1; and Het-2 is defined as inclaim 1; and wherein n, m, p, and q, the same or different, may be 0, 1,or 2; and wherein R₁₃, R₁₄, R₁₅, R₁₆, R₁₇, R₁₈, R₁₉, and R₂₀ are thesame or different and are selected from the group of H, lower alkyl,alkoxyalkyl, aryl, cyano, alkoxycarbonyl, —(CR₂₁R₂₂)_(l)-Het, wherein0<l<10, and R₂₁ e R₂₂, the same or different, are selected from thegroup of H, lower alkyl, and Het may be Het-1 or Het-2 or Het-3, orHet-4.
 19. The compound according to claim 17, having the followingformula (6)


20. The compound according to claim 18, having the following formula (3)


21. The compound according to claim 18, having the following formula (7)


22. A two-photon absorbing chromophore, in solution or in a solid state,being a compound of claim
 17. 23. A two-photon absorbing chromophore, insolution or in a solid state, being a compound of claim
 18. 24. Acompound of general formula (I) according to claim 17 for use intwo-photon absorption systems.
 25. A compound of general formula (I)according to claim 17 for use as optical power limiting agent viatwo-photon absorption.
 26. A compound of general formula (I) accordingto claim 17 for use as imaging agent with two-photon absorbing activityfor application in detection technologies such as two-photon laserscanning fluorescence microscopy.
 27. A compound of general formula (II)according to claim 18 for use in two-photon absorption systems.
 28. Acompound of general formula (II) according to claim 18 for use asoptical power limiting agent via two-photon absorption.
 29. A compoundof general formula (II) according to claim 18 for use as imaging agentwith two-photon absorbing activity for application in detectiontechnologies such as two-photon laser scanning fluorescence microscopy.30. A composition for use in two-photon absorption systems comprising acompound according to claim
 24. 31. A composition for use in two-photonabsorption systems comprising a compound according to claim
 27. 32. Thecomposition according to claim 30 comprising a polymer material chosenamong poly(acrylate), poly(methacrylate), polyimide, polyamic acid,polystyrene, polycarbonate, polyurethane.
 33. The composition accordingto claim 30 comprising an organically-modified silica (SiO₂) network.34. The composition according to claim 32, wherein said compound islinked to the said polymer material by covalent bonds.
 35. Thecomposition according to claim 33, wherein said compound is linked tothe said silica network by covalent bonds.
 36. The composition accordingto claim 30 for use as optical power limiting agent via two-photonabsorption.
 37. The composition according to claim 30 for use as imagingagent with two-photon absorbing activity for application in detectiontechnologies such as two-photon laser scanning fluorescence microscopy.38. A composition according to claim 31 comprising a polymer materialchosen among poly(acrylate), poly(methacrylate), polyimide, polyamicacid, polystyrene, polycarbonate, polyurethane.
 39. The compositionaccording to claim 31 comprising an organically-modified silica (SiO₂)network.
 40. The composition according to claim 38 wherein said compoundis linked to the said polymer material by covalent bonds.
 41. Thecomposition according to claim 39 wherein said compound is linked to thesaid silica network by covalent bonds.
 42. The composition according toclaim 31 for use as optical power limiting agent via two-photonabsorption.
 43. The composition according to claim 31 for use as imagingagent with two-photon absorbing activity for application in detectiontechnologies such as two-photon laser scanning fluorescence microscopy.